Load cell

ABSTRACT

A load cell is disclosed for measuring forces and moments with respect to two or three axes of measurement. A combination of load cell geometry and strain gage arrangement results in the measurement of all quantities as a function of shear stress.

ilnited States Patent [191 Shoberg LOAD CELL [75] Inventor: Ralph S.Shoberg, Farmington,

Mich.

[73] Assignee: GSE, Incorporated, Livonia, Mich.

[22] Filed: May 17, 1972 [21] Appl. No.: 254,117

[52] US. Cl 73/141 A [51] Int. Cl. G011 1/22 [58] Field of Search 73/141A, 133 R;

[56] References Cited UNITED STATES PATENTS 3,427,875 2/1969 Saxl 73/141A Nov. 13, 1973 3,365,689 1/l968 Kutsay 73/141 A X PrimaryExaminer.lerry W. Myracle Attorney-Gerald E. McGlynn, Jr. et al.

[ 5 7] ABSTRACT A load cell is disclosed for measuring forces andmoments with respect to two or three axes of measurement. A combinationof load cell geometry and strain gage arrangement results in themeasurement of all quantities as a function of shear stress.

18 Claims, 7 Drawing Figures PMENTEUHQT' '4 3 197a SHEET 10F 2 T c1 TOPSTRUT ZZ c3 TOP STRUT /5 c2 BOTTOM STRUT XX .04 BOTTOM STRUT A6 (:5 ToPSTRUT Z? c7 TOP sTRuT Zfl I 9 a 1 n. 1

T6,C6 BOTTOM STRUT Z; T8,C8 BoTToMsTRuT Z LOAD CELL INTRODUCTION Thisinvention relates to force transducers, commonly called load cells, andparticularly to a force transducer in which load forces are measuredalong a plurality of axes of sensitivity exclusively as a function ofshear stress.

BACKGROUND OF THE INVENTION Single-axis force transducers are well knownand find application in many situations where load and stress analysisis desired. For example, it is quite common to employ force transducersin the testing of numerous automotive components in environmentsincluding crash tests. A load problem can often be satisfactorilyanalyzed using one or more single axis transducers. In other cases theproper analysis of the stress and load factors involved requires amultiaxis force transducer. The realization of accurate data from amultiaxial device is complicated by interaxial effects; i.e., a tensileload along one axis usually sets up compressive loads along the otheraxes such that the transducer body itself gives rise to an analysiserror. In addition, the strain-sensitive elements in such multiaxialdevices often require very precise and difficult compensation in orderto balance out various other error effects.

BRIEF DESCRIPTION OF THE INVENTION According to the present invention, ahighly-accurate multiaxial force transducer is provided wherein loadforces as well as moments are measured with reference to two or moresensitive axes exclusively by response to shear stress. In accordancewith the present invention, the interaxial effects are, therefore,eliminated in that the transducer is rendered insensitive to bending andtorsion as well as tension and compression in the flexure areas.

In general, this is accomplished by means of a transducer having anintegral body of a high modulus of elasticity material, such as toolsteel or aluminum, the body being effectively divided into at least twomajor portions between which a load is imposed. The two or more majorportions are interconnected by struts of greatly reduced cross-section,these struts constituting the flexure areas. Strain sensitive means,such as strain gauge resistors, are disposed on the struts in such anarrangement and interconnected as to respond only to shear stresses inthe struts. Accordingly, a two-axis device involves a plurality ofstruts extending along or parallel to an axis which is mutuallyperpendicular to the two sensitive axes and a three axis device involvesa plurality of struts, some of which extend along or parallel to oneaxis and others of which extend along or parallel to another axis whichis perpendicular to the first. I

As indicated above, the subject invention is readily embodied in two andthree axis force transducers, a specific and preferred example of eachbeing hereinafter described in great detail. In the preferred form ofeach embodiment, the loads are imposed through plate members which areconnected exclusively to respective major portions of the transducerbody on opposite sides thereof along a central axis. In the illustratedembodiments, the transducer body is essentially disk shaped with thecorresponding surfaces on opposite sides of the body being parallelbutnoncoplanar as between the two or more major portions of the body.Accordingly, one plate member is mechanically secured exclusively to theplane surface or surfaces of one major area on one side of the body andthe other plate member is secured exclusively to the plane surface orsurfaces of the other major body portion on the other side of the body.The plates, thus, act as load transfer mechanisms and, as hereinafterdescribed in greater detail, also serve as mechanical stops to preventstressing of the load cell body beyond the elastic limits ofthematerial.

Another feature of the preferred embodiments of the invention is the useof load transfer plates having perimeter flanges to enhance theresistance of the device to bending or torsional strains.

Another feature of the invention as hereinafter described in greaterdetail is the use of stacked or laminated strain gage resistor pairs,these stacked or laminated pairs being disposed on the fiexure strutsand interconnected in an electrical bridge network which, for each axisof sensitivity, provides accurate compensation for all unwanted forcecomponents including tension, compression, bending, and torsion. Theresult is that the network responds only to shear force on the struts.In general, the resistive bridges preferably comprise four bridge legseach leg including two parallelconnected paths having two seriesresistors in each path. The series connected resistors in each path areof like load type, i.e., tension or compression, and are disposed onaxially opposite sides of the force transducer body. Moreover, theindividual sensitive axes of the resistors in each stacked pair aremutually perpendicular and nonaligned with the longitudinal axis of thestrut on which they are disposed. Accordingly, the application of asuitable dc or ac voltage source across one pair of bridge terminals andthe application of a suitable voltage-responsive meter across the otherpair of bridge terminals results in a highly accurate load analysis byreference toshear stresses and, hence, the complete isolation of eachaxis of sensitivity in the force transducer is accomplished.

Various other features and advantages of the invention will becomeapparent from a reading of the following specification which describestwo specif c and preferred embodiments of the invention in detail.

BRIEF DESCRIPTION OF THE DRAWINGS.

- FIG. 1 is a plan view of a two-axis load cell embodying the invention;I

FIG. 2 is an end view, partly in cross-section, of the force transducerof FIG. 1; v

FIG. 3 is an enlarged'view of a strut in the device of FIG. 1 showingthe orientation of the strain gage resistors thereon;

I FIG. 4 is an electrical schematic diagram of a bridge network for usein connectionwith the strain gage resistors in the force transducer ofFIGS. 1 and 2;

FIG. 5 is a table indicating the relation of the strain gage resistorsin the circuit of FIG. 4 to the struts in the device of FIG. 1;

FIG. 6 is a plan view of a three-axis load cell embodying the invention;and,

FIG. 7 is an end view, partly in cross-section of the force transducerof FIG. 6.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS Looking now to FIGS. 1and 2, a two-axis force transducer is shown to comprise a transducerbody having a generally disk-like shape and being fabricated from solidmaterial having a high modulus of elasticity, such as tool steel oraluminum. The transducer body 10 is a solid, integral unit, but ismachined into a configuration which defines a rigid, major body portions12, 14, and 16 of which body portion 12 lies substantially centrally ofthe body 10 and has the major dimension thereof along an axis identifiedas X-X in FIG. 1. Major body portions 14 and 16 are reversely similarlateral portions and are disposed uniformly on opposite sides of the XXaxis of symmetry of the body 10.

Lateral body portion 14 is integrally joined to 18 and 20 ofsubstantially reduced cross-sectional area and which lie parallel to theaxis identified in FIG. 1 as Y-Y. Similarly, lateral body portion 16 isintegrally joined with the central body portion 12 by means of struts 22and 24, both of which lie parallel to and uniformly spaced about theaxis Y-Y.

The struts 18, 20, 22, and 24 define flexure areas to concentrate thestrain effects of a load imposed between the central body portion 12 andthe two lateral portions 14 and 16 by means to be described. The struts18, 20, 22, and 24 are bored out to provide untapped holes 26 and 28,these holes being representative of similar holes bored through thestruts 22 and 24. It will be observed in FIGS. 1 and 2 that the boredholes extend centrally through the struts and also through portions ofthe adjacent lateral body portions 14 and 16. The purpose of the boredholes is to minimize the cross-sectional area of the struts whilemaintaining substantial bending resistance.

The X and Y axes form part of a triaxial system X, Y, and Z as isclearly identified in FIGS. 1 and 2. The sensitive axes of the forcetransducer illustrated in FIGS. 1 and 2, that is, the axes along whichload forces are measured and about which moments are measured, are the Xand Z axes since loadings along these axes produce shear forces in thestruts 18, 20, 22, and 24. A pure Y axis load imposes only compressiveforces in the struts and, accordingly, is not measurable in accordancewith the system herein described with reference to FIGS. 1 and 2.However, a triaxial system is de-v scribed hereinafter with reference toFIGS. 5 and 6.

Central body portion 12 is provided with smooth, milled, parallel andopposite surfaces 30 and 32. Similarly, lateral body portions 14 and 16are provided with smooth, parallel, and opposite plane surfaces 34, 36and 38, all of the aforementioned surfaces lying in parallel X-Y planes.However, as can be clearly seen in FIG; 2, surface 30 lies abovecoplanar surfaces 34 and 38 along the Z axis on one side of thetransducer body 10 whereas lateral portion surfaces 36 and 40 lie abovethe central body portion surface 32 on the other side of the transducerbody 10. For a typical force transducer having a four inch diameter thedifference in axial elevation of the aforementioned surfaces is on theorder of 0.05 inches.

The force transducer of which transducer body 10 is a part furtherincludes force transmitting plate members 42 and 50 which are connectedto the highest elevation body portion surfaces on the opposite sides ofthe disk-shaped transducer body 10, as best shown in FIG. 2. Platemember 42 is shown to comprise a flat disk of a diameter somewhatgreater than the diameter of the transducer body 10 and having holes 44bored therethrough to accommodate a spaced pair of machine screws whichsecure the plate member to the central body portion 30 of the transducerbody by means of tapped holes 46. Accordingly, the interior surface ofthe plate member 42 abuts and is flush against the elevated surface 30of the central body portion 12 and is spaced from the surfaces 34 and 38of the lateral body portions 14 and 16 by approximately 0.05 inchesalong the Z axis. Force adapter plate member 42 is formed with aperimeter flange 48 to increase the stiffness thereof and to resistbending.

Plate member 50 is substantially similar to plate member 42 and isprovided with holes 52 which cooperate with tapped holes 54 in the planesurfaces of lateral body portions 14 and 16 to permit the plate member50 to be screwed to the transducer body 10. When in position, the innerplane surface of the plate member 52 abuts and is flush against theplane surfaces 36 and 40 of the lateral familiar portions 14 and 16 andis spaced from the surface 32 of the central body portion byapproximately 0.05 inches. Plate member 50 is may provided with aperimeter ways 56 for increased stiffness. may

To apply a load to the force transducer, plate 50 may be suitablyanchored to one element of a forceproducing system and plate member 42may be suitably anchored to another element of a force-producing system.The introduction of a force along the X axis between plate members 42and 50 is transmitted from plate member 42 to the central body portion30 and thence through struts 18, 20, 22, and 24 to the lateral bodyportion 14 and 16 and thence to the anchored plate member 50.

It will be readily appreciated by those faimilar with load cells andsimilar devices that the geometry, configuration and proportion of thetransducer body 10, as well as the plate members 42 and 50 mau bemodified in many wasy to accommodate different specific environments.For example, the force transducer body 10 ma be fabricated in a square,rectangular, or other polygonal configuration. Similarly, the struts 18,20, 22, and 24 need not be coplanar or arranged exactly as shown in thedrawing as various modifications are possible.

Looking now to FIG. 3, a preferred arrangement of strain gage resistors58 and 60 on the strut 18 is illustrated. In FIG. 3, the surface ofstrut 18 which appears is the same surface as appears in the plan viewof FIG. 1 and this illustration is to be taken as representative of thepreferred strain gage resistor configuration on each of the struts 18,20, 22, and 24 in the two-axis device of FIG. 1. Similarly, strain gagearrangements corresponding to that described with reference to FIG. 3are also found on the surfaces of struts 18, 20, 22, and 24 which areopposite those visible in the plan view of FIG. 1 such that eightindividual pairs of strain gage resistors are employed in theimplementation of the X axis strain gage system of FIGS. 1 and 2. Aswill be described hereinafter, another set of eight pairs of resistorsis employed in the implementation of the Z axis measurement system.

Referring more specifically to FIG. 3, a laminated pair of strain gateresistors 58 and 60 is shown to be disposed on the surface of strut 18so as to respond to the shear stress which is produced in the strut 18by loading along either the X axis. Strain gage resistor 58 is aconventional strain-sensitive rosette suitably bonded to the surface ofthe strut 18 with the principal axis of sensitivity thereof disposed at45 to the Y axis; i.e., exactly half-way between the X and Y axes, asillustrated in FIG. 3. Strain gage resistor 60 is disposed immediatelyover strain gage resistor 50, but is oriented with the principal axis ofsensitivity thereof at 90 to the axis of sensitivity of strain gageresistor 58. The two strain gage resistors are laminated over oneanother by means of an insulating material, such as varnish or epoxy.

As previously mentioned, a second laminated pair of mutually orthogonalstrain gage resistors is disposed on the side opposite the side of strut18 which is visible in FIGS. 1 and 3. Accordingly, a total of fourstrain gate resistors, i.e., two pairs, is disposed on strut 18.Similarly, two pairs of stacked laminated strain gage resistors aredisposed on each of struts 20, 22, and 24 making a total of sixteenstrain gage resistors on the struts 18, 20, 22, and 24 for the purposeof monitoring forces having components along the X axis.

Referring now to FIGS. 4 and 5, the connection of the sixteen straingage resistors on the struts 18, 20, 22, and 24 for measurement offorces along the X axis is shown. As will be apparent to those skilledin the strain measurement art, the network 61 illustrated in FIG. 4 is avariation on the well-known Wheatstone bridge which is commonly used tomeasure variations in electrical resistance The bridge network 61comprises legs 62, 64, 66, and 68 each of which comprises a pair ofparallel-connected, resistive paths. In leg 62, for example, path 70comprises individual series-connected resistors T1 and T2 while theparallel-connected path 72 comprises the series combination ofindividual resistors T3 and T4. Similarly, leg 64 comprisesparallelconnected paths 74 and 76 of which path 74 comprises the seriesconnection of strain gage resistors C1 and C2, while path 76 comprisesthe series combination of strain gage resistors C3 and C4.

The table of FIG. 4 is a key to the location of the individual resistorsin the bridge network 61 in the physical arrangement of the transducerbody 10, shown in FIGS. I and 2. In this arrangement it is to beunderstood that each individual resistive element, such as T1, T2, C1,C2, in FIG. 4 corresponds to an individual strain gage resistor orrosette, such as 58 and 60, as disposed on the struts 18, 20, 22, and 24of the transducer body of FIGS. 1 and 2.

Each individual strain gage resistor is nominally of the same value,e.g., 350 ohms, and therefore, each leg 62, 64, 66, and 68 has a totalnominal resistance of 350 ohms when the transducer body 10 is in theunstrained condition. The individual strain gage resistors identified asT1 and C1 in the bridge network 60 of FIG. 4 constitute a firstlaminated pair and are disposed on the top of strut 22; that is, themajor surface of strut 22 which is in the plane of the paper in thedrawing of FIG. 1. Similarly, individual resistive elements T3 and C3comprise a second laminated pair of resistors mounted -on the top ofstrut 18. In other words, the resistors T3 and C3 in the network 60ofFlG. 4 correspond with resistors 58 and 60 in the drawing of FIG. 3.The location of the remaining resistors in the bridge network 60 of FIG.4 is clearly indicated by the table adjacent FIG. 4 in the drawings.

A dc source 78 of between 10 and volts is connected between terminals 80and 82 of the bridge network 60 while a volt meter 84 is connectedbetween terminals 86 and 88 of the bridge network. Thus, as a variationin resistance occurs in one or more of the legs 62, 64, 66, and 68 ofthe bridge network 60, an unbalance condition occurs which produces areading on the volt meter 84. This reading may be readily calibratedinto force readings so as to indicate the quantity of force applied tothe transducer body 10 along the X axis.

It can be seen that for a load applied to the transducer body 10 alongthe Y axis of FIG. 1 all of the strain gage resistors disposed on thestruts 18, 20, 22, and 24 experience similar components of stress and,accordingly, experience similar quantities of resistance variation.Therefore, every element in the bridge network of FIG. 4 changescorrespondingly and the bridge network remains balanced. The same effectobtains for a pure Y axis compressive loading and, accordingly, thebridge network 60 is insensitive to Y axis loadings which invariableproduce tensile and compression forces.

For Z axis loads, one side of each strut along the Z axis. is placed incompression and the other side is placed in tension. However, it can beseen from the diagram and table of FIG. 4 that each series resistancepair, such as T1 and T2, comprises an individual strain gage resistorofwhich one is on the top and the other is on the bottom of a strut and,hence, as one resistive element increases in value, the other decreasesby a corresponding amount. Accordingly, Z axis loadings have no effectin unbalancing the bridge network 60 of FIG. 4. A similar analysisobtains for torsional and bending loads on each of the struts 18, 20,22, and 24.

Accordingly, the bridge circuit network 60 of FIG. 4 in operativeassociation with the physical arrangement of the transducer body 10 ofFIGS. 1 and 2 produces a force transducer which is purely responsive toshear stresses and, by virtue of the overall arrangement, exclusivelyresponsive to a load imposed along the X axis.

The other axis of sensitivity of the force transducer illustrated inFIGS. 1 and 2 is the Z axis, as previously mentioned. To implement the Zaxis system, a second bridge network substantially identical to bridgenetwork 60 of FIG. 4 is established. Again, the struts 18, 20, 22, and24 constitute the support system for the strain gage resistors and,again, laminated pairs of strain gage resistors are employed. However,the laminated pairs are mounted on the struts at locations which aredisposed at 90? around the strut, i.e., rotated around an axis parallelto the Y axis, as shown in FIG. 1. A representative pair 90, 92 ofindividual strain gage resistors disposed in a stacked and laminatedcombination is shown on the strut 20 of FIG. 2. Again a total of sixteenindividual strain gage resistors is employed to constitute theseries-parallel Wheatstone bridge measurement network which is requiredto implement the Z axis load-sensitive system in such a fashion as to beinsensitive to tensile and compressive loadings as well as torsional andbending loads and, thus, to produce a system which is exclusivelyresponsive to Z axis loads which produce a shear stress in the struts18, 20, 22, and 24.

Referring now to FIGS. 6 and 7, a three-axis load cell or forcetransducer is shown. The three-axis force transducer again comprises atransducer body 94 of generally disk shape and fabricated as an integralunit from high modulus'of elasticity material, such as tool steel oraluminum. Transducer body 94 is formed with a central bore 96 whichextends along and is centered about an axis of symmetry, identified inFIGS. 6 and 7 as Z-Z. Four uniformly-spaced radial cuts extending fromthe bore 96 effectively divide the body 94 into four major sectors 98,100, 102, 104 of large crosssectional area and consequential highrigidity. A transverse axis X-X passes commonly through the majorsectors 98 and 102 and is perpendicular to the axis of symmetry Z-Z.Similarly, an axis Y-Y passes commonly through the major sectors 100 and104 and is mutually perpendicular to both the X and Z axes. Accordingly,the X, Y, and Z axes designated in FIGS. 6 and 7 are useful not only indescribing the geometry of the body 94, but also define the sensitiveaxes of the force transducer.

Transducer body 94 is also machined in such a fashion as to define fourminor sectors 106, 108, 110, and 112. Minor sector 106 is joined to themajor sector 98 by means of a strut 114 of substantially reducedcrosssectional area and to major sector 100 by means of a strut 116.Similarly, minor sector 108 is joined to major sector 100 by means ofstrut 118 and to major sector 102 by means of strut 120. Minor sector110 is joined to major sector 102 by means of strut 122, and the majorsector 104 by strut 124. Finally, minor sector 112 is joined to majorsector 104 by means of strut 126 and to major sector 98 by means ofstrut 128. It is apparent in FIG. 6 that struts 114, 120, 122, and 128are all parallel to the Y axis while struts 116, 118, 124, and 126 areall parallel to the X axis. Accordingly, a comparison of the transducerbody 94 in FIG. 6 to the twoaxis transducer body of FIG. 1 indicates thepresence of a second set of four parallel struts and, hence, providesthe basis for the third axis of measurement.

As best shown in FIG. 7, the surfaces 130 of the major sectors 98 and102 are of lesser height than the coplanar surfaces 132 and 134 of themajor sectors 104 and 100, respectively, measured along the X axis.Conversely, the coplanar surfaces 136 of the major sectors 98 and 102are higher in elevation than the coplanar surfaces 138 and 140 on theopposite sides of the transducer body measured along the Z axis. Thisdifference in axial elevation facilitates the connection ofa flangedplate member 142 exclusively to the surfaces 132 and 134 on the upperside of the transducer body 94, as shown in FIG. 7, and facilitates theconnection of the flanged plate member 144 exclusively to the surfaces136 of the major sectors 98 and 102 on the opposite side of thetransducer body.

In addition to the machine screws illustrated in FIG. 7 for the purposeof mechanically connecting the plate members 142 and 144 to therespective major sector surfaces, a pair of alignment pins 146 aredisposed on the inner surface of the plate member 142 and extend alongthe Z axis to be received into corresponding sockets or cavities 148 inthe transducer body 94. Similarly, locator pins or alignment pins 150depend from the inner surface of plate member 144 and enter sockets orcavities 152 in the transducer body.

By virtue of the plate member interconnection to opposite pairs of majorsectors on opposite sides of the transducer body 94, as shown in FIG. 7,load may be transferred from one plate member to the other through thetransducer body in such a fashion as to produce flexure in the struts.By strain gage resistor arrangement hereinafter described, the X, Y, andZ components of these forces may be separately and independentlyidentified solely by measurement of shear stress.

As indicated in FIG. 6, each strut carries on the upper surface thereofa laminated pair of strain gage resistors having mutually orthogonalaxes of sensitivity and being disposed on a diagonal, that is, at to theaxis of the strut on which it is placed. Strain gage resistor pair 156disposed on strut 122 is representative. Similarly, stacked pairs ofstrain gage resistors are mounted on all eight of the struts on the sideopposite that shown in FIG. 6. Looking to FIG. 7, strain gage resistorpair 158 mounted on the underside of strut 122 is representative.Finally, a third set of stacked, laminated strain gage resistor pairs ismounted on the side surfaces of struts 124, 126, 116, and 118, as bestshown in FIG. 6. Strain gage resistor pair 162 on strut 124 isrepresentative.

The electrical interconnection of the strain gage resistor pairs into anelectrical bridge network of the type shown in FIG. 4 is readilyaccomplished with each of the three sets of sixteen individual straingage resistors found on the transducer body 94 of FIGS. 6 and 7. Thestrain gage resistor pairs mounted on struts 114, 120, 122, and 128 areconnected together to form the X axis bridge network. Theinterconnection follows the identical pattern as was followed for struts22, 24, 20, and 18, respectively, in interconnecting the strain gageresistors in the X axis bridge netwrok of FIGS. 1 and 4. The individualdetails of this interconnection will, therefore, not be repeated. Aseparate and independent Y axis bridge is established by interconnectingthe strain gage resistors on the struts 116, 118, 124, and 126 in anidentical fashion. Finally, a Z axis bridge is established byinterconnecting the strain gage resistors on the side surfaces of struts116, 118, 124, and 126 in an identical fashion. The result is three,separate and independent bridge networks each responsive only to forcecomponents along the respective axis of sensitivity and each respondingonly to shear. Complete compensation against the sensation of tension,compression, bending and torsion is accomplished in the fashiondescribed with reference to FIG. 1, 2 and 4.

The force transducer of FIGS. 6 and 7, like the force transducer FIGS. 1and 2, can also be employed as a torque sensor. To adapt the device to atorque sensor from the linear axis-force sensor previously described, aminor reconnection of strain gage elements is required. However, nophysical modification of the force transducer body 94 or the location ofthe individual strain gage resistors on the body is required.

To measure torques or moments about the X and Y axes the strain gageresistors for measuring Z axis forces are employed. However, because theshear stress on one side of the axis of sensitivity is opposite indirection to the shear stresses in the struts on the opposite side ofthe axis of sensitivity rather than in the same direction, the bridgenetworks are rewined by reversing the polarities on one half of thebridge. This has the effect of cancelling the Z axis force and addingthe .two shear force measurements on the opposite side of the axis ofsensitivity to produce a pure torque indication.

To measure a moment or torque about the Z axis, a

set of sixteen individual strain gage resistors on four selected strutsare interconnected into a bridge circuit so as to measure tension in thestruts at the corners defined by the minor sectors 108 and 112 and tomeasure the compression at the struts interconnecting the minor sectors106 and 110. A straightforward electrical interconnection of the bridgecircuit elements is believed to be apparent.

In summary, it has been shown that a simple, effective, andhighly-accurate multiple-axis force transducer has been disclosedwherein all force measurements are made by measuring shear forces in theflexure struts interconnecting the major transducer body portions. Thebridge interconnection described herein is selfcompensating againstminor resistor errors as well as against bending, torsion, tension, andcompression forces. The subject device iseasily accommodated to a widerange of forces, for example, from 2,000 pound per axis to 50,000 poundper axis. The device finds many uses including the measurement of forcedistribution in bumpers, steering columns, steering wheels, trailerhitches,crash barriers, mannequins, and vehicle wheels. It will beapparent to those skilled in the art that many modifications from thedevice described herein are possible and, accordingly, the foregoingspecification is not to be construed in a limiting sense.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A force transducer comprising: a solid, substantially disk-shapedbody of high modulus of elasticity material, said body includingopenings formed therein to define at least first and second rigid andintegral major portions interconnected by relatively short shear strutshaving substantial bending resistance and being of reducedcross-sectional area, each of said first and second rigid major portionshaving opposite and parallel plane surfaces, said openings extendingbetween said surfaces, a first plate member exclusively connected to oneof the plane surfaces of the first portion and spaced from thecorresponding plane surface of the second portion, and a second platemember exclusively connected to the opposite surface of the secondportion and spaced from the corresponding surface of the first portion;and strain sensitive means disposed on the struts and connectedexclusively to monitor shear in the struts as a consequence of forcesapplied to said body through said plate members along at least two axesof said body.

2. A force transducer as defined in claim 1 wherein the first and secondbody portions are substantially coplanar sections of said disk-shapedbody, but the plane surfaces of the first and second portions are offsetalong the axis of symmetry by substantially less than the axialthickness of said body so as to be non-coplanar, the surface of thefirst portion being higher along said axis than the surface of thesecond portion on one side of the body, the surface of the secondportion being higher along said axis than the surface of the firstportion on the other side of the body to facilitate the exclusiveconnections of the plate members thereto.

3. A force transducer as defined in claim 2 wherein saidstrain-sensitive means comprises laminated pairs of strain-gageresistors disposed on orthogonally oriented sides of the struts with theaxes of sensitivity of the resistors in each pair being mutuallyperpendicular, and at least one electrical bridge network includingselected ones of said resistors connected to define four legs, each leghaving two parallel paths of two resistors each, the resistors of eachpath being disposed on opposite sides of the same strut, and theresistors of one path being on a different strut than the resistors ofthe parallel path associated therewith to render the bridge networkelectrically non-responsive to bending loads.

4. A force transducer as defined in claim 2 wherein each of said platemembers comprises a flat disk overlying the plane surface of the bodyportion to which it is connected, and a perimeter flange to stiffen theplate member to resist bending under load.

5. A two-axis force transducer comprising: a solid, body of high modulusof elasticity material and having a transverse axis of symmetry, saidbody including a rigid central portion and two reversely similar, rigid,lateral portions spaced from the central portion about said axis andbeing joined to said central portion by pairs of struts of substantiallyreduced cross-section, said struts being parallel and perpendicular tosaid axis, each of said central and lateral portions having opposite,parallel plane surfaces, a first plate member connected exclusively tothe plane surface of said central portion on one side of the body, asecond plate member connected exclusively to the plane surfaces of thelateral portions on the other side of the body, said strainsensitivemeans on said struts to monitor shear in the struts as a consequence offorces applied to aid body through the plate members.

6. A force transducer as defined in claim 5 wherein the body issubstantially disk-shaped, the opposite plane surfaces of the centralportion are non-coplanar with the corresponding plane surfaces of thelateral portions, the plane surface of the central portion being abovethe corresponding lateral portion plane surfaces on one side of the bodyand below the corresponding lateral portion plane surfaces on the otherside of the body to facilitate the exclusive connections thereto and toprovide a mechanical stop during flexure of the body.

7. A force transducer as defined in claim 6 wherein each of said platemembers comprise a flat disk overlying the plane surface of the bodyportion to which it is connected, and a perimeter flange to stiffen theplate member to resist bending under load.

8. A force transducer as defined in claim 5 wherein said strainsensitive means comprises laminated pairs of strain gage resistorsdisposed on said struts and means connecting the resistors into anelectrical bridge network.

9. A force transducer as defined in claim 5 wherein each of the strutsis rendered hollow by a bore extend ing longitudinally therethrough andthrough an adjacent lateral body portion.

10. A force transducer as defined in claim 5 wherein each of the platemembers carries at least one locator pin extending therefrom, and arecess in each of the central and lateral body portions to receive alocator pin therein.

11. A three-axis force transducer comprising: a solid, disk-shaped bodyof high modulus of elasticity material and having an axis of symmetry;said body including four major sectors arranged uniformly about theaxis, four minor sectors disposed between the major sectors, andorthogonally arranged pairs of struts connecting the minor sectors tothe adjacent major sectors, the major and minor sectors being physicallyseparated from one another in the body except for the interconnection ofsaid struts, each of the major sectors having parallel and oppositeplane surfaces, a first plate exclusively connected to the planesurfaces of a first diametrically opposite pair of major sectors on oneside thereof, a second plate exclusively connected to the plane surfacesof the other diametrically opposite pair of major sectors on the otherside thereof, and strainsensitive means on said struts to monitor shearin the struts as a consequence of forces applied to said body throughthe plates.

12. A force transducer as defined in claim 11 wherein the plane surfacesof a first diametrically opposite pair of major sectors lie above thesurfaces of the second pair of sectors on one side of the body, and liebelow the surfaces of the second pair on the opposite side of the body,the plates being connected to the higher surfaces on the opposite sidesto produce shear forces in the struts when loaded.

13. A force transducer as defined in claim 11 wherein each of the platesis a flat disk of greater diameter than the transducer body and having aperimeter flange for stiffening purposes.

14. A force transducer as defined in claim 1 1 wherein each of theplates includes a pair of pins extending therefrom on opposite sides ofand parallel to the axis of symmetry, each of the major sectorsincluding a cavity into which a plate pin fits to secure the plates tothe transducer body.

15. A force transducer as defined in claim 1 1 wherein the strainsensitive means includes strain-gage resistors disposed on the struts.

16. A force transducer as defined in claim 15 wherein the resistors arearranged in laminated pairs on opposite sides of the struts, thesensitive axes of the resistors in each pair being mutually orthogonaland non-aligned with the longitudinal axis of the strut on which thepairis disposed, the resistors on the four struts parallel to one transverseaxis being connected into a first shear sensitive bridge network and theresistors on the four struts parallel to the other transverse axis beingconnected into a second shear sensitive bridge network.

17. A force transducer as defined in claim 16 wherein the first bridgenetwork comprises four major legs, each leg including two parallelconnected paths having two series-connected resistors in each path, theresistors in each path being selected from opposite sides of a strut,all of the resistors in each leg being of like tensile-compressiveresponse to load forces applied to the body along a selected axes ofmeasurement, thereby to render the entire network sensitive exclusivelyto shear forces along an axis through the body perpendicular to thelongitudinal axis of the struts on which the network resistors aredisposed.

18. A force transducer as defined in claim 17 including additionalstrain-gage resistors on the surfaces of the struts parallel to the axisof symmetry, and means for connecting the resistors into a bridgenetwork for measuring shear in the struts as a consequence ofa loadbetween said plates along the axis of symmetry.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,771, 359Dated November 13, 1973 inventofls) Ralph S. Shoberg, Farmington,Michigan It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

In the Specification: v Column 4, lines 20 to 23, delete "Plate member50 is may provided with a perimeter ways 56 for increased stiffness.may" and insert therefor Plate member 50 is also provided with aperimeter flange 56 for increased stiffness.

Column 4, line 36, delete "man" and insert may-.

line 37, delete "wasy" and insert -ways--. line 39, delete "ma" andinsert may-.

Column'S, line 12, delete "gate" and insert -gage- Column 6,: line 17,delete "invariable" and-insert -invariably-.

Column 7, v

line 36, "X axis" should be Z a-xis-. Column 8, line 27, delete"netwrok" and insert -network-.

line 56,'delete "rewined" and insert rewired-.

Signed and sealed this 10th day of September 1974.

(SEAL) Attest: MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting OfficerCommissioner of Patents FORM PC4050 (10-69) I uscoMM-oc 60376-P69 Q U.S.GOVERNMENT PRINTING OFFICE: i9 84

1. A force transducer comprising: a solid, substantially diskshaped bodyof high modulus of elasticity material, said body including openingsformed therein to define at least first and second rigid and integralmajor portions interconnected by relatively short shear struts havingsubstantial bending resistance and being of reduced cross-sectionalarea, each of said first and second rigid major portions having oppositeand parallel plane surfaces, said openings extending between saidsurfaces, a first plate member exclusively connected to one of the planesurfaces of the first portion and spaced from the corresponding planesurface of the second portion, and a second plate member exclusivelyconnected to the opposite surface of the second portion and spaced fromthe corresponding surface of the first portion; and strain sensitivemeans disposed on the struts and connected exclusively to monitor shearin the struts as a consequence of forces applied to said body throughsaid plate members along at least two axes of said body.
 2. A forcetransducer as defined in claim 1 wherein the first and second bodyportions are substantially coplanar sections of said disk-shaped body,but the plane surfaces of the first and second portions are offset alongthe axis of symmetry by substantially less than the axial thickness ofsaid body so as to be non-coplanar, the surface of the first portionbeing higher along said axis than the surface of the second portion onone side of the body, the surface of the second portion being higheralong said axis than the surface of the first portion on the other sideof the body to facilitate the exclusive connections of the plate membersthereto.
 3. A force transducer as defined in claim 2 wherein saidstrain-sensitive means comprises laminated pairs of strain-gageresistors disposed on orthogonally oriented sides of the struts with theaxes of sensitivity of the resistors in each pair being mutuallyperpendicular, and at least one electrical bridge network includingselected ones of said resistors connected to define four legs, each leghaving two parallel paths of two resistors each, the resistors of eachpath being disposed on opposite sides of the same strut, and theresistors of one paTh being on a different strut than the resistors ofthe parallel path associated therewith to render the bridge networkelectrically non-responsive to bending loads.
 4. A force transducer asdefined in claim 2 wherein each of said plate members comprises a flatdisk overlying the plane surface of the body portion to which it isconnected, and a perimeter flange to stiffen the plate member to resistbending under load.
 5. A two-axis force transducer comprising: a solid,body of high modulus of elasticity material and having a transverse axisof symmetry, said body including a rigid central portion and tworeversely similar, rigid, lateral portions spaced from the centralportion about said axis and being joined to said central portion bypairs of struts of substantially reduced cross-section, said strutsbeing parallel and perpendicular to said axis, each of said central andlateral portions having opposite, parallel plane surfaces, a first platemember connected exclusively to the plane surface of said centralportion on one side of the body, a second plate member connectedexclusively to the plane surfaces of the lateral portions on the otherside of the body, said strain-sensitive means on said struts to monitorshear in the struts as a consequence of forces applied to aid bodythrough the plate members.
 6. A force transducer as defined in claim 5wherein the body is substantially disk-shaped, the opposite planesurfaces of the central portion are non-coplanar with the correspondingplane surfaces of the lateral portions, the plane surface of the centralportion being above the corresponding lateral portion plane surfaces onone side of the body and below the corresponding lateral portion planesurfaces on the other side of the body to facilitate the exclusiveconnections thereto and to provide a mechanical stop during flexure ofthe body.
 7. A force transducer as defined in claim 6 wherein each ofsaid plate members comprise a flat disk overlying the plane surface ofthe body portion to which it is connected, and a perimeter flange tostiffen the plate member to resist bending under load.
 8. A forcetransducer as defined in claim 5 wherein said strain sensitive meanscomprises laminated pairs of strain gage resistors disposed on saidstruts and means connecting the resistors into an electrical bridgenetwork.
 9. A force transducer as defined in claim 5 wherein each of thestruts is rendered hollow by a bore extending longitudinallytherethrough and through an adjacent lateral body portion.
 10. A forcetransducer as defined in claim 5 wherein each of the plate memberscarries at least one locator pin extending therefrom, and a recess ineach of the central and lateral body portions to receive a locator pintherein.
 11. A three-axis force transducer comprising: a solid,disk-shaped body of high modulus of elasticity material and having anaxis of symmetry; said body including four major sectors arrangeduniformly about the axis, four minor sectors disposed between the majorsectors, and orthogonally arranged pairs of struts connecting the minorsectors to the adjacent major sectors, the major and minor sectors beingphysically separated from one another in the body except for theinterconnection of said struts, each of the major sectors havingparallel and opposite plane surfaces, a first plate exclusivelyconnected to the plane surfaces of a first diametrically opposite pairof major sectors on one side thereof, a second plate exclusivelyconnected to the plane surfaces of the other diametrically opposite pairof major sectors on the other side thereof, and strain-sensitive meanson said struts to monitor shear in the struts as a consequence of forcesapplied to said body through the plates.
 12. A force transducer asdefined in claim 11 wherein the plane surfaces of a first diametricallyopposite pair of major sectors lie above the surfaces of the second pairof sectors on one side of the body, and lie below the surfaces of thesecond pair on the opposite side of the body, the plates being connectedto the higher surfaces on the opposite sides to produce shear forces inthe struts when loaded.
 13. A force transducer as defined in claim 11wherein each of the plates is a flat disk of greater diameter than thetransducer body and having a perimeter flange for stiffening purposes.14. A force transducer as defined in claim 11 wherein each of the platesincludes a pair of pins extending therefrom on opposite sides of andparallel to the axis of symmetry, each of the major sectors including acavity into which a plate pin fits to secure the plates to thetransducer body.
 15. A force transducer as defined in claim 11 whereinthe strain sensitive means includes strain-gage resistors disposed onthe struts.
 16. A force transducer as defined in claim 15 wherein theresistors are arranged in laminated pairs on opposite sides of thestruts, the sensitive axes of the resistors in each pair being mutuallyorthogonal and non-aligned with the longitudinal axis of the strut onwhich the pair is disposed, the resistors on the four struts parallel toone transverse axis being connected into a first shear sensitive bridgenetwork and the resistors on the four struts parallel to the othertransverse axis being connected into a second shear sensitive bridgenetwork.
 17. A force transducer as defined in claim 16 wherein the firstbridge network comprises four major legs, each leg including twoparallel connected paths having two series-connected resistors in eachpath, the resistors in each path being selected from opposite sides of astrut, all of the resistors in each leg being of liketensile-compressive response to load forces applied to the body along aselected axes of measurement, thereby to render the entire networksensitive exclusively to shear forces along an axis through the bodyperpendicular to the longitudinal axis of the struts on which thenetwork resistors are disposed.
 18. A force transducer as defined inclaim 17 including additional strain-gage resistors on the surfaces ofthe struts parallel to the axis of symmetry, and means for connectingthe resistors into a bridge network for measuring shear in the struts asa consequence of a load between said plates along the axis of symmetry.